It is known from U.S. Pat. No. 3,490,975 to provide an artificial artery or vascular graft by winding extruded silicone rubber thread onto a mandrel so as to form a porous tube. The tube then is cured and the cured tube is removed from the mandrel. The curing step is required to achieve fibre-to-fibre bonding and to form a stabilized structure, in view of the nature of the material used to form the tube. In a typical example of this procedure, thread of 180 to 190 micron thickness is used at a helix angle of 35.degree. to the axis of the mandrel to produce a product having a porosity of 80%. Such fibre size provides side wall openings which are too large to permit proper use of the product as a vascular graft.